View Full Version : Reverse reaction - how to avoid
01-03-2005, 01:27 PM
Can the biodiesel reaction be reversed? This question appears now and then. To check it out I carried out a reaction and allowed the temperature to continually rise. I used new oil with 7.2g KOH and 210ml methanol per litre of oil. The reaction mixture was above 70 degrees for 12 minutes before any methanol distilled off, which should have been ample time to achieve a high level of conversion at this temperature.
The temperature was allowed to rise further until all the excess methanol had been collected, but no more. The temperature was 117deg C at this point. After cooling and separating the ester layer was found to still contain 1% free methanol, proof of reverse reaction.
A large amount of the dreaded white stuff was observed during washing. The final yield, washed and dried, was only 72%. Over a quarter of the product had been lost. The glycerine layer was heavier than expected but not enough to account for the missing esters. Clearly a considerable amount was lost during washing.
One can conclude that the reaction readily goes in reverse but the distribution of product is not the same as in the forward reaction. Most of the loss must be mono and diglyceride as if it were triglyceride it would restore the weight of the upper layer. It would be necessary to use GC to determine exactly how the material was distributed.
It appears that boiling off some methanol is a more efficient way to produce the dreaded white stuff than merely skimping on reagent.
01-03-2005, 03:36 PM
Now if we can just find a use for the dreaded white stuff, we can use this more efficient process for making it, and increase our profit. http://biodiesel.infopop.cc/groupee_common/emoticons/icon_wink.gif
Another good experiment.. Sounds like mark's unintended reverse reaction experiment of a few weeks ago, only under controlled settings.
I'm not surprised that you'd end up with mostly mono and diglycerides, instead of back-reacting all the way to triglycerides.. Its probably easier to esterify a glycerol to a single FFA than it is to add an FFA to a diglyceride.
Maybe if the reaction were left ton continue for longer, you'd end up with a higher percentage of triglycerides, which would be a handy way to make cooking oil out of biodiesel, in case you were short on it around the house.
01-03-2005, 06:33 PM
yep, I seen it with my very eyes!
Were you stirring during this?
01-04-2005, 02:16 AM
Who, me? Would I stir?
It helps distillation if you stir. Even with stirring I was surprised to find how hot it was necessary to get biodiesel to distil off methanol. Even at 115deg C there was still methanol coming off. And if you are trying to dry oil that contains water it is positively dangerous if you don't stir because of the risk of "bumping".
Tony from West Oz (The Wizard of Oz)
01-04-2005, 11:44 AM
Another Excellent Experiment.
I do have a suggestion for the next time you do this.
Take a sample of the3 stages:
- The oil,
- The biodiesel before methanol recovery (reaction reversal),
- The final product.
Have the samples tested to determine glyceride content (Mono, Di, Tri - or simply total glycerides).
This will quantify initial reaction conversion and the degree of reversal (That even the great Biogod would not be able to dispute).
01-06-2005, 02:57 PM
One of the ways that has been suggested for stopping the reaction so that the methanol can be distilled off in the presence of the lower layer is to add a little water to destroy the catalytic effect of the caustic.
I tried this, adding 5% water to the hot reaction when it should have been complete, then continuing to raise the temperature and boil off the methanol. The amount of methanol boiled off was restricted to about 80% of the excess to avoid pushing the reverse reaction too hard.
Even so the eventual yield, washed and dried, was 90%, much better than obtained previously without water, but still well down on the 98% by weight obtained on the same oil by the normal process, separation, washing and drying.
This time there was no evidence of excessive dreaded white stuff, as happened without water, so the loss must be soap and proves that these conditions cause saponification of the esters as one would expect. There was still almost 1% methanol in the esters so some methanol must have come from the esters, further proof of saponification.
Perhaps it would work better done at a low temperature using vacuum.
So adding water does not prevent the loss of product, it just makes it different.
10-06-2006, 03:13 PM
I'll post on how it works with a vacuum, goin in tonight.
10-10-2006, 06:02 AM
In you first posting on this page could you please explain how you determined that a reversal of the reaction occured just because there was 1% methanol left in the esters after cooling and seperation?
surely having reaction of 12 minutes is not sufficient to get a full conversion anyway
How was the reaction done ie was it in a mixing unit, bottle shake, or other method, I would think that depending on any one of these could effect the reaction compleatness without giving rise to reaction reversal just because the methanol has been distilled off.
Also what level of ffa content was in the methanol that was distilled off?
Why would boiling off the methanol cause more dreded white stuff than washing out the methanol or just letting the methanol come off under normal time evaporation? Surely the amount of DWS would be the same in the product after the reaction, after all is is laden with soaps which would be consentrated in the DWS by the distilation of the methanol because they need somewhere to go. Adding water just provides another medium for the soaps to migrate to when the meth is removed.
Im confused by your results
Neutral hasn't posted since 24 feb 2006 1.31am. Sadly. His inupt is much missed and his 3070 postings have been a significant contribution to say the least.
I hope he is well wherever he is but fear we may not hear from him again. I for one would like to though.
So don't take it personally if you get no reply.
10-24-2006, 03:11 AM
I dont take it personal but I suggest he is envolved with a project and unless you post and find on this site regular it can get lost.
maybe we should just keep this on the front page so he can find it.
there again I might just ring him
Strangely but gladly he's back.
10-25-2006, 03:20 AM
>: "In you first posting on this page could you please explain how you determined that a reversal of the reaction occured just because there was 1% methanol left in the esters after cooling and seperation? Surely having reaction of 12 minutes is not sufficient to get a full conversion anyway"
It had a lot longer than 12 minutes. It probably took about 20 minutes going from 50C to 70C, then 12 minutes above 70C. Plenty of time. 1% methanol in the esters is quite a lot after distilling it off so hard. There would have been about double that weight of methanol in the glycerine also.
> "How was the reaction done ie was it in a mixing unit, bottle shake, or other method, I would think that depending on any one of these could effect the reaction compleatness without giving rise to reaction reversal just because the methanol has been distilled off."
Reaction done in a 3 neck round bottom flask on hot plate with magnetic stirrer. Well Stirred. There is no question that this reaction did not go properly. It was done in a way which has worked properly many times before. So the fact that I observed so much white stuff on washing and obtained such a low yield is absolute proof of reverse reaction.
> "Also what level of ffa content was in the methanol that was distilled off?"
FFA doesn't distill off.
> "Why would boiling off the methanol cause more dreded white stuff than washing out the methanol"
Boiling off the methanol will not cause more dreaded white stuff than washing out the methanol provided the lower layer is removed first. The purpose of this expt was to see whether you could safely recover excess methanol from both the esters and the glycerine simultaneously. It proved you could not. The dreaded white stuff was clearly manufactured by reacting the methyl esters with the glycerol while the methanol was boiled off, ie reverse reaction.
would the fact it was well stirred and heated promote the reverse reaction?
With less stirring and lower tempratures using vacuum distallation could one not perhaps extract the methanol without the reverse reaction taking place?
10-27-2006, 07:31 AM
apologies for the duplicate post but after answering the question in the other thread I thought the answer really belonged in this thread.
There is a consensus that the backward reaction will not happen if you recover from the bio and the glycerine seperately.
There is a consensus that it is possible to have a backward reaction from recovering from the bio and glycerine together. For example if you mix hard and keep pouring heat in to keep the temp up.
There is disagreement on whether it is possible to recover from both together without the backward reaction happening.
If using minimal circulation to avoid the glycerine foaming and vacuum distillation down to 2psi absolute pressure then there is enough heat in the mix post production to provide for the latent heat of vapourisation for all the exess methanol. This means that you do not have to add extra heat.
Under these conditons of a rapidly cooling system with minimal circulation and perhaps with the air displacement in place at the top, then it is possible that you may be able to perform the distillation with no or insignificant reverse reaction taking place.
Minty uses a similar method happily without knowing if he has a reverse reaction or not. Opinion is again divided on if he does from the information given.
But you would have to try it to be sure.
03-07-2007, 03:31 AM
I am recovering methanol from the biodiesel after draining glycerine from my appleseed reactor. However, due to the somewhat imperfect draining of our water heater, there must be some residual glycerine remaining in the reactor when we turn the heat back on. In addition, I think there is some more byproduct settling out after the initial draining.... when I went to drain the de-methed biodiesel this morning there was solid glycerol/ soap in stuck in the drain.
OK anyway my question... do you think that having ANY byproduct in the reactor during methanol recovery phase will cause a reverse reaction? Argh!
vBulletin® v3.7.4, Copyright ©2000-2013, Jelsoft Enterprises Ltd.